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Launched by NASA on board the Canadian satellite SCISAT in 2003, the Atmospheric Chemistry Experiment (ACE) was intended for a two-year mission. Fifteen years later, ACE is still providing excellent spectra which provide vital chemical and physical data about our atmosphere, particularly the ozone layer.

“ACE monitors the global distribution of more than 35 different species including CFCs, hydrochloric acid, and ozone - in other words, nearly all molecules specified by the Montreal Protocol and associated with the Antarctic ozone hole,” says Peter Bernath, ACE Mission Scientist and team lead for ACE’s Science Operations Centre headquartered in the Department of Chemistry. “ACE represents quite an achievement in terms of return on investment, both for science and policy.”

The 1987 Montreal Protocol has been hailed as the world’s most successful international environmental treaty. It phases out the production of chlorofluorocarbons (CFCs) and other substances shown to deplete Earth’s protective ozone layer. ACE is monitoring the decline of these banned source gases in the lower atmosphere and of product gases such as hydrochloric acid in the stratosphere.

“We’re the only ones in orbit doing this, and in real time as a function of altitude,” says Bernath. “You can actually watch the ozone hole forming on our website where we post near-real time data every day.”

ACE is not only known for monitoring the ozone hole; scientists worldwide have published more than 430 papers using ACE data sets. For example, ACE data were used to show how the Asian monsoon directly injects combustion-generated pollution into the upper atmosphere by tracking hydrogen cyanide gas produced mainly by fires.

ACE data were also used to prove solar activity acts as an additional source of atmospheric nitrous oxide in the upper atmosphere. Previously, the only known natural source of nitrous oxide was denitrifying bacteria living in soils at the Earth’s surface. Nitrous oxide is not only an important greenhouse gas; it’s also a powerful ozone-depleting molecule.

The main ACE instrument was built in Quebec City by ABB and it has vastly outperformed the Canadian Space Agency’s original requirements. However, the satellite itself has no fuel on board. Its orbit is decaying steadily by about 1 kilometre per year. It’s scheduled to burn up in the atmosphere by 2035.

Meanwhile, this science mission continues to evolve and improve. The high resolution spectroscopic data gathered by ACE allows the Waterloo ACE team to forensically identify new species and then quantify their global concentration trends going back the entire 15 years of the project.

“We recently received a request to provide data on two hydrofluorocarbons (HFCs) that are being regulated as part of the 2016 Kigali Amendment to the Montreal Protocol,” says Bernath. “We were able to isolate the signal in the spectra and within a month provide an entirely new data product. The unique algorithms the Waterloo team continue to develop are only possible with this project’s longevity.”

The Waterloo team is also producing cloud data for the upper troposphere, stratosphere and mesosphere. ACE is the first spectrometer to record spectra of polar mesospheric clouds (PMCs), showing these clouds are in fact very small ice particles.

PMCs form at about 90 km above the ground when temperatures are very low. Climate change causes surface temperatures to increase and the upper atmosphere to cool, increasing the occurrence of PMCs. ACE data are expected to provide additional information on climate change.

The Journal of Quantitative Spectroscopy and Radiative Transfer will be publishing a Special Issue in honour of the 15th anniversary of ACE’s launch. For more information and to view the data freely available to the public, visit the ACE website, http://www.ace.uwaterloo.ca/. Additional research products are available upon request.

Peter Bernath is a Research Professor in the Department of Chemistry at Waterloo, professor and Eminent Scholar at Old Dominion University in Norfolk, Virginia, and professor emeritus at University of York in Great Britain. ACE’s Science Operations Centre includes (photo, back row left to right) Chris Boone, Scott Jones, Johnny Steffen, (front row) Dennis Cok and Peter Bernath.